Geology Cafe

Landslide Hazards—An Awareness Guide

Table of Contents

Landslides - Where, When, And Why Do They Occur?

  • Landslides are associated with hilly or mountainous landscapes. They are also common along coastlines and river valleys.
  • Landslides occur most frequently in regions where climate and precipitation, bedrock and soil conditions, and slopes are susceptible to failure.
  • Within the United States, landslide-prone regions include the Western Coast Ranges, the Cascades volcanic region, large portions of the central and northern Rocky Mountain region, the Appalachian region, and in many areas along the Mississippi River and its tributaries (below is a map of landslide hazard-prone areas within the United States).
  • Major disasters associated with landslides have occurred in many other regions of the world including the Andes Mountains of South America, through the volcanic island chains of the southern and western Pacific Basin (throughout the region known as the “Ring of Fire”), and in the tropical regions of Central America. Avalanches occur in all high mountainous regions of the world—too often with deadly results. Many of these landslide, debris flow, and debris flood events are associated with tropical storms (hurricanes and typhoons) and volcanic eruptions.
Landslides are mostly likely to occur in areas where they have already occurred in the past. In many cases, the landscape features surrounding a location where recent landslide catastrophes have occurred provide evidence of past and ongoing landslide activity. Landsliding is part of the processes behind the evolution of the landscape. The history of landslide activity in any region through time can often be deciphered on scales ranging from decades to many thousands or even millions of years.
landslides map of the US
Landslide-prone areas within the United States . For more detailed maps of selected regions and map legend information see USGS Open-File Report 97-289.

Click here to see a landslide hazards map of California.

Factors that make an area prone to landsliding include the following characteristics:

  • Slope—Long, steep hillsides, cliffs, escarpment, or rugged mountainous region are prone to landslides.
  • Rapid erosion—Wave erosion constantly cuts away materal from the base of sea cliffs. Rapid down-cutting and meandering stream channels undermine slopes. Moving glaciers carves oversteepened slopes. Glaciers can also collapse under their own weight causing landslides that can travel for miles down slope.
  • Landslide-prone bedrock conditions—This includes: 1) steeply-dipping of layered sedimentary rocks, 2) soft rock (particularly shale), 3) poorly consolidated sediments (such as recently uplifted or exposed marine sediments, river floodplain deposits, or glacial deposits), 4) loose volcanic (pyroclastic) deposits, or 5) deeply weathered bedrock such as what commonly exists in region with warm, humid climates, or 6) highly fractured bedrock such as along large, active fault zones. Often several of these factors can apply to the same area.
  • Seasonally wet periods—Heavy precipitation initiates landsliding, particularly in association with flood conditions. However, landslide is more likely to occur later in a wet when the cumulative seasonal precipitation has saturated the ground. Soil slopes and rock are weakened through saturation by heavy rains or snowmelt.
  • Mountain climatic conditions—Higher regions are prone to freeze-thaw cycles, and wetter condition promote organic activity and associated weathering; winter snowstorms may produce avalache-prone conditions.
  • Human activity—Landslides are frequently caused by the undercutting of slopes during road construction or when fill materials are dumped onto already unstable or marginally stable slopes. Improperly constructed mine tailing piles, dams, and waste landfills have resulted in landslides, sometimes with catastrophic results.
  • Biological factors—Plant cover protects soil from erosion. Conversely, because of the lack of plant cover in desert regions, infrequent desert storms frequently result in debris flows. Grazing, lumbering, removal of chaparral, and wildfires remove vegetative cover and create the potential for landsliding during wet weather that follows.
  • Landslide trigger mechanisms—Landslides are triggered by earthquakes, major storms, volcanic activity, or other natural or human-induced activities that may cause the earth to move. The additional weight of storm rains or snow melt can cause slopes to fail or reactivate older landslides. Earthquakes create stresses that make weak slopes fail.
Because landslides take a variety of forms and occur in a variety of setting, the key to predicting when they might occur or how they form and may evolve throught time. The study of existing landslides in a region will provide a sense of what bedrock and landscape conditions will make an area susceptible to landsliding, and historical accounts of past landslide activity may provide insight into when landslides are more likely to occur. In many cases, heavy rains set off landslides. However, in many cases landslides may also occur in unpredictable was, such as the timing of a rockfall after a spring thaw. Sometime deep-seated slumps or mudflows will not move until days or weeks have passed after a storm. Casastrophic failures may be as unpredictable as an earthquake that might trigger them.

Because of the unpredictable nature of of landslides, public safety and preparedness education should focus on understanding the cause and character of landsliding, recognition of tell-tale conditions that indicate landsliding might occur, and awareness on what to do when landslide activity occurs.